Multicolor electrothermic recording sheet

ABSTRACT

A multicolor electrothermic recording sheet which comprises a support having thereon a layer of a metal compound which can be electrically reduced to a metal or a lower valency metallic compound in a heat sensitive element layer comprising a combination of (1) a polyhydroxy compound or a weak acid compound and (2) a color forming leuco dye which undergoes a color forming reaction upon contact with the polyhydroxy compound or the weak acid compound and a high molecular weight binder, whereby the metal compound is electrically reduced by electrothermic energy to form a metallic color and the leuco dye is thermally developed.

United States Patent [191 Shimuzu [451 Feb. 4, 1975 1 MULTICOLORELECTROTHERMIC RECORDING SHEET [75] Inventor: Kazuo Shimuzu, Tokyo,Japan [73] Assignee: Mitsubishi Paper Mills, Ltd., Tokyo,

Primary Examiner-J0seph W. Hartary Attorney, Agent, or FirmSughrue,Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT A multicolor electrothermicrecording sheet which comprises a support having thereon a layer of ametal compound which can be electrically reduced to a metal or a lowervalency metallic compound in a heat sensitive element layer comprising acombination of (1) a polyhydroxy compound or a weak acid compound and(2) a color forming leuco dye which under goes a color forming reactionupon contact with the polyhydroxy compound or the weak acid compound anda high molecular weight binder, whereby the metal compound iselectrically reduced by electrothermic energy to form a metallic colorand the leuco dye is thermally developed.

7 Claims, 1 Drawing Figure MULTICOLOR ELECTROTHERMIC RECORDING SHEETBACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This inventionrelates to a multicolor electrothermic recording shee't whereinmulticolor recorded images can be formed by utilizing electrothermicenergy and by controlling the signals provided by such energy.

2. DESCRIPTION OF THE PRIOR ART Hitherto, a method for obtainingelectric recording sheets wherein color images are formed by electricalenergy is known wherein recorded images are formed on the free metalbackground having a different color from that of the recorded images bya dry type of electrolysis of a metal compound as disclosed in JapanesePatent Publication No. 2234l/63. In the case of forming monochromaticrecorded images, such a conventional method is suitable because odorlessstable images having a comparatively high density are obtained withoutthe necessity for a high electrical voltage as in an electricdischarging recording paper or electrostatic recording paper.

However, in such electrolytic recording papers, it is difficult toobtain distinguishable multicolor recorded images having two or morecolors, because the hues of the images are limited by the metal even iftwo or more metal compounds are used together so as to obtain metalimages having a different hue from each other. Further, it is necessaryto strictly control the electric voltage electric current for theelectrolysis. Accordingly, such a-method is not practically used.

On the other hand, recently it has been necessary to obtain recordingsheets which can be recorded at a high speed without carrying out anytreatment and can form multicolor recorded images having ahigh contrast.

In order to meet such requirements and desires for obtaining distinctmulticolor recorded images, attempts have been made to combine theelement wherein metallic developed images are formed by electricallyreducing a metal compound to a free metal or a lower valency metalcompound, such as the above described electrolytic recording elem ent,with a heat sensitive element wherein color images are formed thermallyand preferably at a temperature of above 100C. Thus, the production ofmulticolor images having a high density at a high speed by controllingthe electric voltage and the electric current have been successful.

SUMMARY OF THE INVENTION In the electric recording device used for thepresent invention, the recording sheet is brought into contact with arecording needle through which an electric current passes by means ofthe circuit device which can control the electric voltage and theelectric current so as to provide a definite electrolysis value to therecording sheet, and thus thermographic development is carried out dueto the .Ioule heat of the electric energy. If necessary, it is possibleto change a very low electric energy into a heat energy in an intrinsicsensitive range using two recording devices.

It is also possible in another embodiment of the invention to developthe recording element by controlling the resistance using a resisterwhile the electric current or the electric voltage is set at a definitevalue by a closed circuit.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING The drawing is an obliqueview of an example of the recording process of the present invention.wherein I is a power source. 2 is a resister, 3 is a recording sheet. 4is a conductive layer, 5 is an clectrothermic layer, 6 is a recordingneedle and 7 is a ground.

DETAILED DESCRIPTION OF THE INVENTION In the present invention, it isnecessary that the metal compound be stable at a temperature of above200C, and thus the heat sensitive element generally causes a colorforming reaction at a temperature in the range of above I00 to 200C. Itis also possible that both of the metal compound and the heat sensitiveelement are subjected to reacting at the same condition.

Accordingly, thermographic color formation does not compete with thegray black color formation of the metal compound. Further, it ispossible to obtain each of the color images even if both of the elementsare mixed together, because both image forming materials developed arenot compatible with each other. In this case, if a high molecular weightbinder is carbonized itself, further excellent metallic color formationcan be carried out.

Namely, thermographic color formation is usually carried out at a lowertemperature and metallic color formation is carried out at a highertemperature. Further, the resulting images are not only nontransparentbut also have a conductivity such as carbon black because the highmolecular weight binder is carbonized. Consequently, mixing of bothcolors hardly occur because a deep gray black color is formed andbleeding of images which has been often resulted in the priorheat-sensitive elements containing two materials having a differentmelting point hardly occur in the present invention.

In the following, each component used in the present invention will beillustrated.

The requirements desired for the metal compound used in this inventionare as follows: l it must be possible to electrically reduce the metalcompound to a metallic compound having a different color, (2) the metalcompound should be a white or a light color and should be insoluble inwater and certain kinds of organic solvents (e.g. petroleum solvents),and (3) the metal compound must be comparatively conductive. Of course,the metal compound should be non-toxic and stable.

Examples of metal compounds are zinc oxide, lead oxide, barium sulfate,lithopone, basic lead carbonate and tin oxide. However, zirconium oxide,titanium oxide, bismuth oxide, silver oxide, nickel oxide, vanadiumoxide, antimony oxide, aluminium oxide and calcium oxide, which do notsatisfy all of the above described requisites completely, can also beused, if desired.

These metal compounds can be used alone or as a mixture of two or morethereof. Further, the metal compounds can be mixed with a high molecularweight binder or mixed with an aluminum powder, a zinc powder orstainless steel flakes.

Suitable examples of heat sensitive elements which can be used are thosedescribed in Japanese Patent Publication Nos. 4160/1968 and 3680/1969and U.S. Pat. Nos. 3,45l,338 and 3,239,366.

Namely, as polyhydroxy compounds, those compounds having two hydroxygroups are preferably used. The melting point of these compoundsbasically controls the temperature of thermographic color formatron.

Examples of suitable polyhydroxy compounds include 4,4'-isopropylidenediphenol, 4,4-secbutylidene-bis-(2-methylphenol), 4.4-cyclohexylidene-bis-(Z-methylphenol), 2.2-

dihydroxydiphenyl, 2,2'-methylene-bis-(4- fluorophenol), halogenatedphenol resins, novolak phenol resins and a or B-naphthol, etc. Examplesof weak acid compounds are benzoic acid, succinic acid, tartaric acid,citric acid, guanidine nitrate, urea nitrate, boric acid, carbonic acidand other inorganic solid acids (e.g., Kaolin, Bentonite, acidic clay),etc. However, the weak acids are not limited to these compounds. Theyare added, if desired, in order to control the temperature of colorformation.

Typical examples of the leuco dye compounds (especially containing alactone ring) include Crystal Violet lactone, benzo leuco MethyleneBlue, Malachite Green lactone, Rhodamine B-lactam,benzoindolinospiropyran, 3-diethylaminc-7-dibenzylaminofluoran,3-diethylamino7-(N-methylanilino)-fluoran, spiro-( 1-benzopyran-2,2'-naphthopyran), 3-diethylamin0-6- methoxyfluoran,3,3-bis-(p-diethylaminophenyU- phthalide andl,3,3-trimethyl-6'-chloro-8'- ethoxyindolino-benzospiropyran. Thesecompounds may be used as a mixture of two or more thereof in order toform a different color. It is necessary to add a high molecular weightbinder to these leuco dye compounds. Typieal examples of suitable highmolecular weight binders include gelatin, polyvinyl alcohol,hydroxyethylcellulose, gum arabic, casein, carboxymethylcellulose,methylcellulose and polyvinyl methyl ether-maleic acid anhydridecopolymers for an aqueous medium and terpene, cyelized rubber, petroleumresins, acrylic rubber, styrene-butadiene resins, alkyl methacrylateresins, vinylidene chloride resins and nitrocellulose for an organicsolvent medium. These resins sometimes change into black carbonizedproducts at the carbonization temperature by controlling the electriccurrent electric voltage. The composition of these compounds dependsupon the thickness of the coating film, conditions of the recordingdevice and kind of dispersion medium. However, the following compositionis usually used. Hereinafter, all percentages, ratio and the like are byweight unless otherwise indicated.

Basic Leuco Dye 100 (base) Polyhydroxy Compound 50 to 1000 MetalCompound 50 to 500 High Molecular Weight 10 to 500 Binder A decrease inthe amount of the high molecular weight binder sometimes causes acollapse of the coating layer and an increase thereof interferes withthe color forming reaction. Therefore, the amount of the high molecularweight binder should be determined according to the conditions adopted.

As the dispersion medium, water or petroleum is particularly preferred.Alternatively, materials which dissolve only the high molecular weightbinder, such as toluene, xylene and ligroin, can be used.

In the present invention, other components such as wax, paraffin, talc,microfine particles of polymers, metal powders, starch. glass powders,calcium carbonate, clay, carbon black, carbon fibers, Orange Yellow,guanidine, polyoxyalkylene derivatives, polyol compounds, surface activeagents, ultraviolet ray absorbing agents, fluorescent brighteningagents, defoaming agents, hardening agents, etc. can be added thereto.As the support of the sheet, a conductive paper treated so as to have aresistivity below 10 Q/cm thermoplastic films having a vacuum evaporatedfilm of copper iodide, aluminium or zinc, and metal foils can be used.

In .the present invention, the recording device shown in the appendeddrawing can be used. As the power source, either a direct current sourceor an alternative current source can be used. However, a direct currentsource is more preferred.

A device having a variable resister in a closed circuit so as to controlthe electric current electric voltage and a device which can control theintrinsic reduction value of the metal compound by increasing theelectric current electric voltage can be used. Such devices aredescribed in U.S. Pat. No. 3,299,433 and Japanese Patent Publication No.22341/1963.

The electrothermic recording sheets of the present invention can beused, for example, for computor outputs, printing typewriters electriccalculators, tachometers, automatic recording devices, facsimiles,oscillography, reproduction of television images, automatic drawinginstruments, astronomical observation recorders and electrocardiograms,etc.

in the following, the present invention will be illustrated in greaterdetail byreference to the examples. However, the scope of the presentinvention is not to be interpreted as being limited to these examples.Unless otherwise indicated, all parts, percents, ratios and the like areby weight.

EXAMPLE 1 A sheet of paper having a weight of 40 g/m waslaminated withan aluminium foil having a thickness of 1.5 microns. A coating solutionhaving the following composition was applied to this sheet in athickness of 5 u.

A ZnO g Terpene Resin 20 Petroleum (kerosine) (boiling point, about200C) 13 Crystal Violet lactone l2 Terpene Resin (Piccolite, 30 producedby Esso Co.)

Petroleum (kerosine) 500 B 4,4'-lsopropylidene-diphenol 45 Tcrpene Resin2O Petroleum 250 (Ratio ofA, B and B is 5:121)

Using the resulting recording sheet, recording was carried out under thefollowing conditions in the same manner as that shown in the appendeddrawing.

Electric Voltage: 100 V A Phthalic Acid Anhydridc l 3 A blue colorappeared. Polyethyl Methacrylate 7 Electric Current: 80mA 5 Toluene 50Electric Voltage: 70 V B Li i 80 A black p Polyethyl Mcthacrylatc l5Electric Current: 40mA 5 phcnyl)phthalide C Aluminum Flakes 5 Theelectric current electric voltage condition de- 10 ggf g 'sf pends uponthe film thickness, the ratio of each compo- 5mm ("mic name r Asuhi Downent and the type of the recording device, etc. Chemical Compa y.vinylidcnc chloride vinyl chloride 8:2) EXAMPLE 2 Xylvnv A B C l 2 5 Adispersion having the following composition was 15 applied as a firstlayer in a thickness of 3.5 microns to a white film sheet comprisingpolycarbonate (50 p.) and cupric iodide (5 1.) prepared by vacuumevaporation which wastreated so as to be conductive (l().3

Q/cm

A Ligroin I00 g Styrene-Butadienc Resin (8:2) l2.5 (Dow Latex No. 620,produced by Dow Chemical Company) 4,4-lsopropylidene-diphenol 12.5

B Ligroin 100 Styrene-Butadiene Resin l2.5 (as defined above) N PhcnylRhodamine B Lactam 12.5

To the coated layer, the following coating solution was applied in athickness of microns.

Zinc Oxide 40 g Barium Sulfate 40 Polymethyl Methacrylate Resin 2OToluene 100 EXAMPLE 3 Zinc was deposited by evaporation (5O 120A) on apolyester film having a thickness of 70 microns, and then a coatingsolution having the following composition was applied to the resultingzinc layer so as to form a recording layer having a thickness of 3.5microns.

In this recording sheet, a brownish black image was formed at 160V and25mA, a deep blue image was formed at V and 70mA, and a nearly blackimage -was formed at V and 50mA.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:

l. A multicolor electrothermic recording sheet which comprises a supporthaving thereon a layer of a metal compound which can be electricallyreduced to a metal or a lower valency metallic compound in a heatsensitive element layer comprising a combination of (l) a polyhydroxycompound or a weak acid compound and (2) a color forming leuco dye whichundergoes a color forming reaction upon contact with said polyhydroxycompound or said weak acid compound and a high molecular weight binder,whereby said metal compound is electrically reduced by electrothermieenergy to form a metallic color and said leuco dye is thermallydeveloped.

2. The recording sheet of claim 1, wherein said polyhydroxy compound isa dihydroxyphenol.

3. The recording sheet of claim 1, wherein said weak acid compound is anorganic acid.

4. The recording sheet of claim 1, wherein said leuco dye is a leuco dyecontaining a lactone ring.

5. The recording sheet of claim 1, wherein said metal compound is acompound ofa metal in a low oxidation state.

6. The recording sheet of claim 5, wherein said metal compound is anoxide or hydroxide.

7. The recording sheet of claim 6, wherein said metal compound is zincoxide, lead oxide, antimony oxide,

zinc hydroxide. barium hydroxide, or barium sulfate.

1. A MULTICOLOR ELECTROTHERMIC RECORDING SHEET WHICH COMPRISES A SUPPORTHAVING THEREON A LAYER OR A METAL COMPOUND WHICH CAN BE ELECTRICALLYCOMPOUND IN A HEAT SENSITIVE ELEMENT LA ENCY METALLIC COMPOUND IN A HEATSENSITIVE ELEMENT LAYER COMPRISING A COMBINATION OF (1) A POLYHYDROXYCOMPOUND OR A WEAK ACID COMPOUND AND (2) A COLOR FORMING LEUCO DYE WHICHUNDERGOES A COLOR FORMING REACTION UPON CONTACT WITH SAID POLYHYDROXYCOMPOUND OR SAID WEAK ACID COMPOUND AND A HIGH MOLECULAR WEIGHT BINDER,WHEREBY SAID METAL COMPOUND IS ELECTRICALLY REDUCED BY ELECTROTHERMICENERGY TO FORM A METALLIC COLOR AND SAID LEUCO DYE IS THERMALLYDEVELOPED.
 2. The recording sheet of claim 1, wherein said polyhydroxycompound is a dihydroxyphenol.
 3. The recording sheet of claim 1,wherein said weak acid compound is an organic acid.
 4. The recordingsheet of claim 1, wherein said leuco dye is a leuco dye containing alactone ring.
 5. The recording sheet of claim 1, wherein said metalcompound is a compound of a metal in a low oxidation state.
 6. Therecording sheet of claim 5, wherein said metal compound is an oxide orhydroxide.
 7. The recording sheet of claim 6, wherein said metalcompound is zinc oxide, lead oxide, antimony oxide, zinc hydroxide,barium hydroxide, or barium sulfate.